1. Field of the Invention
The present invention relates to a vibration control apparatus, a lithography apparatus, and a method of manufacturing an article.
2. Description of the Related Art
In a lithography apparatus designed to transfer or form ultra-fine patterns, vibration transmitted from a floor, on which the apparatus is mounted, to the apparatus causes degradation of overlay accuracy or resolution (transfer) performance. Thus, in the conventional lithography apparatus, a vibration isolation apparatus supports a main body of the apparatus to reduce an influence of the floor vibration.
The conventional vibration isolation apparatus includes a gas spring that supports the main body (i.e., a vibration control base). Further, a velocity feedback control system which is constituted with an acceleration sensor that detects an acceleration of the vibration control base and an actuator that applies a force on the vibration control base, damps the vibration. However, even when damping of vibration is performed by constituting the velocity feedback control system, a natural frequency of the vibration isolation apparatus, which depends on a natural frequency of the gas spring, becomes 3 to 5 Hz at lowest. In order to remove vibration to even lower frequency, it is necessary to lower the natural frequency of the vibration isolation apparatus.
A vibration isolation apparatus discussed in Japanese Patent No. 4083708 includes a reference object and a support unit for supporting the object, and constitutes a reference object support system having a natural frequency lower than that of a gas spring. Then, by performing feedback control on a position of the vibration control base with reference to a position of the reference object in the reference object support system, the vibration isolation apparatus having the natural frequency lower than that of the gas spring is realized.
In Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2007-522393, an active vibration isolation (insulation) system is discussed as a conventional art. A ceiling is located at a height zc. A mass 1 is suspended from the ceiling by a spring 1. An actuator is positioned between the ceiling and the mass 1 so as to control a height z2 at which the mass 1 is positioned. A mass 2 is suspended by a spring 2 from the mass 1. The mass 2 is located at a height z3. A sensor detects a distance “d” between the mass 1 and the mass 2. The distance d serves as a gauge of a difference between the height z2 and a height z3, i.e., z2−z3. The sensor generates a feedback signal to a controller, and the controller generates a control signal for the actuator based on the feedback signal. In this process, the sensor, the spring 2, and the mass 2 form a seismograph, and the mass 2 serves as a reference mass or an inertia mass of the seismograph. Tansmissibility of z2/zc can be improved by feeding back a distance signal d to the controller. Therefore, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2007-522393 discusses the fact that dependence of operation of the mass suspended from the ceiling on operation of the ceiling can be reduced by feeding back the distance signal related to a distance between the mass 1, and another inertial reference mass 2. Then, a displacement of a particular mass with respect to the reference mass 2 is measured.
In Japanese Patent No. 4083708, the natural frequency of the reference object support system is lower than the natural frequency of the gas spring. To realize such a low natural frequency, it is necessary to increase a mass of the reference object or to lower rigidity of the spring. However, increasing the mass of the reference object is not favorable in terms of a space necessary for its arrangement. On the other hand, lowering the rigidity of the spring is also unfavorable in terms of difficulty or costs in manufacturing.
The conventional art discussed in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2007-522393 is the active vibration isolation system that performs vibration isolation with respect to a reference object in a seismograph, and does not discuss a vibration control apparatus that controls vibration of a target object like the vibration isolation apparatus. Therefore, any configuration of the active vibration isolation system which is favorable for being applied to such the vibration control apparatus is not discussed.